<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>鼠标移入下划线展开</title>
    <style type="text/css">
        #underline{
            width: 200px;
            height: 50px;
            background: #ddd;
            margin: 20px;
            position: relative;
        }
        #underline::after{
            content: "";
            width: 0;
            height: 5px;
            background: blue;
            position: absolute;
            top: 100%;
            left: 50%;
            transition: all .8s;
        }
        #underline:hover::after{
            left: 0%;
            width: 100%;
        }
    </style>
</head>
<body>
    <div id="underline"></div>
</body>
</html>